Oscillating Power Supply....WTH?

[EchoWars]

Originally posted by routhun
I know its a very old and almost dead thread, but I have question to EchoWars. Before fixing the oscillation problem his comment on this AMP is not to take even if some one gave it for free. I would like to ask him whether he changed his opinion after he fixed the oscillation problem. Why I am asking is I got this amp from some one in exchange for my Kyocera R-861 receiver + Denon DRA-1000 preamp. Currently the amp works but I would like to replace the caps and speaker connectors and zobel filter mod etc before using is as my main amp. Is this amp worth the effort? If not I would like tear down the driver boards and build an high power amp based on LME49810 chip kit. I like the cool meters and hefty heat sinks of the amp. I agree with EchoWars Philips tried to use the cheapest (except CA3100) available part to build this amp. It's amazing how these amps are working even after 30+ years.

Thanks,
Routhun

I probably said that out of frustration. Once the thing is sorted out, it works and sounds pretty good. It's just a bear to work on, and an oddball design.

Move the output Zobel, and replace the poly caps with high-quality non-inductive types. The Zobel worked really well with a .1µf and a 5.1 ohm 3W resistor. You can also replace the input opamp with whatever you like, but when I did the above amp 4+ years ago, I had a bunch of opamps here, and I thought the 5532 sounded best for this amp.

Edit: ...and yes, in that drawing in post #2 has an error...that cap in the Zobel filter is 0.1µf, not 1µf.

[GK]

Oh, and here is where I got the complete service manual for the 7070 from:

http://www.eserviceinfo.com/download...nsui_7070.html

[AndrewT]

Originally posted by ilimzn
Just recently I've had my hands under the hood of an old Sansui 7070 receiver. And guess what, it has the same zobel error - the zobel is at the output terminal, after the output inductor. This precisely defeats the purpose of the zobel network - it is supposed to be a 'default' load at HF, which it certainly cannot be through a coil - except at the resonance frequency of the coil and cap. Further, the resistor in the zobel is a wirewound, which creates another resonant circuit (fortunately at very high frequency) at which the zobel becomes ineffective. Needless to say I will modify that...

Originally posted by G.Kleinschmidt
OK, I just found the Sansui 7070 service manual on the web. The output network is as you describe, but with a 4.7 ohm resistor in parallel with the coil. The coil is labelled as 2.5mH, but I’ll assume that is an error and the coil is 2.5uH.
The Zobel R is also 4.7 ohms and the Zobel C is 100nF.
This network does indeed provide a smooth, low impedance high frequency load to the amplifier output.
Unloaded at HF beyond the resonant frequency it will look like a purely resistive 9.4 ohm load (the sum of the two 4.7 ohm resistors in series). Note that the most favoured Zobel resistance is 10 ohms.

I have attached a sim of the network below. R3 is added to give the source a 9.4 ohm output impedance and the frequency response at “OUT” is plotted.
As can be seen the resonant frequency is at about 300kHz, at which the network provides the lowest impedance and after that the attenuation rises to –6dB and the phaseshift reverts to zero degrees, indicating the network is 9.4 ohms resistive.

This of course assumes that the resistors are not inductive

Thiele was not daft.

[jbateman]

I think that putting a wirewound resistor in series with a cap to form a zobel damping network is wrong. The wirewound has inductance and creates a resonant circuit in a place you really don't want it.

Carbon composition resistors are the right part to use there. Unfortunately, I don't think they are made anymore, although you can sometimes find them through surplus and nos dealers.

Also it is debatable as to whether the output terminal of the amp should be the feedback point or not...depends on the circuit, really... although in this case it helped fix your problem.

[GK]

Originally posted by AndrewT



Thiele was not daft.

It's pretty simple really. At HF the inductor goes open circuit and the capacitor goes short circuit...............

[ilimzn]

Originally posted by G.Kleinschmidt
I have attached a sim of the network below... This of course assumes that the resistors are not inductive

Sadly, they are not. The one in parallel with the coil is, the one in the zobel is not, and in fact neither is the capacitor in the zobel...

[routhun]

Thanks EchoWars!!!

I took out the driver boards from this amp, they need good cleaning, capacitor replacement and Zobel filter mod. This amp is pretty heavy and philips used cheap and thin metal chasis. Only the heatsinks holding this amp together. If heatsinks are removed the front, back and bottom parts are not connected, they were held together by the heat sinks. I wish they used littlebit strong metal bars to held the chasis. If anyone wants to remove the heatsinks to remove driver boards just remove one heatsink at a time and put back this hint sink before removing the other. Otherwise it is pretty nasty to put together because of this nasty heavy power transformer.

Thanks,
routhun

[sakis]

once i had to fix a double deck cassete player from philips ....

of course you need torx and some other special tools to remove things from it
then of course you need three working hands to do that since two are not enough
and of course there is point when clik sound appears and your shop is full of coils springs and plastics .....

philips R&D ha ha ha